Meaning: 'Comprises' means 'consists of' or 'is made up of', indicating the components that form the sleep cycle.

Wrong options:

• formalizes: Formalizes means to make something official or formal, which does not fit the context of describing the components of the sleep cycle.
• initiates: Initiates means to begin or start something, which is not appropriate here as the sentence describes what the sleep cycle consists of, not what it starts.
• speaks: Speaks refers to verbal communication, which is irrelevant in this context as the sentence is about the structure of the sleep cycle.

Correct answer: referred

Explanation: The word 'referred' is used correctly here to indicate that the last stage of sleep is commonly known as REM sleep, fitting grammatically as it is in the past participle form that aligns with the passive construction of the sentence.

Meaning: 'Referred' means to mention or designate something by a specific name, indicating that this stage is known as REM sleep.

Wrong options:

• differed: Differed means to be unlike or distinct, which does not fit the context of naming a stage of sleep.
• addressed: Addressed means to speak to or deal with a topic, which is not relevant in this context as it does not pertain to naming or identifying the stage.
• given: Given means to provide or present, which does not fit the grammatical structure needed to indicate how the stage is known.

Correct answer: organised

Explanation: The word 'organised' fits grammatically as it describes how the stages are arranged or structured, using the past participle form that aligns with the passive voice of the sentence.

Meaning: 'Organised' means arranged or structured in a systematic way, indicating how the stages of the sleep cycle are set up based on brain activity changes.

Wrong options:

• recognised: Recognised means to identify or acknowledge something, which does not fit the context of describing how the stages are arranged.
• stirred: Stirred means to move or mix, which is not relevant in this context as it does not pertain to the arrangement of stages.
• linked: Linked means connected or associated, which does not convey the idea of systematic arrangement as effectively as 'organised'.

Correct answer: irritability

Explanation: The word 'irritability' fits grammatically as a noun that describes a state of being easily annoyed or angered, which aligns with the context of symptoms exhibited due to REM deprivation.

Meaning: 'Irritability' refers to a state of being easily provoked to anger or annoyance, indicating a common symptom of REM sleep deprivation.

Wrong options:

• emotions: Emotions is a broad term referring to feelings, which does not specifically indicate a symptom of REM deprivation as clearly as irritability does.
• desirability: Desirability refers to the quality of being wanted or wished for, which is unrelated to the symptoms of sleep deprivation.
• love: Love is an emotional state or feeling, which does not fit the context of describing symptoms related to sleep deprivation.